1. Field of the Invention
The present invention relates to a pneumatic cleaning system and more particularly to such a system for cleaning debris from particulate bulk material such as raisins, seeds, nuts and the like.
2. Description of the Prior Art
It is known to clean debris from a particulate bulk agricultural product by passing the uncleaned product along a substantially horizontal shaking screen having perforations of a size such that selected or desired articles pass over the perforations while particles of debris, which are substantially smaller than such articles, fall through the perforations. Such a screen effectively removes debris, such as small stones, which are heavier and smaller than the selected articles. It is evident that such a screen, which utilizes the fact that particles of debris are relatively smaller than desired articles, cannot separate debris which has a larger dimension than the desired articles. Further, since conventional shaking screens rely on gravity for separation, it is evident that separation is more effective for debris which has a specific gravity substantially greater than that of the articles of the desired product. It is also evident that other well-known cleaning methods, such as flotation, cannot separate debris which has a lower specific gravity, from the desired product of a higher specific gravity.
It has long been known to clean debris having a substantially lower specific gravity from a desired particulate product of a greater specific gravity by applying a stream of air to the uncleaned material. Such separation is effective when the desired product has a substantially higher specific gravity and is of relatively regular and streamlined form, as when separating chaff from grain. However, such separation is, of course, substantially less effective when the debris and the product are more nearly alike in form, in resistance to air flow, and/or in specific gravity.
An example of the deficiencies of such shaking screen separation is found in raisin processing where stems and substandard raisins are intermixed by nature with raisins of the desired quality. The stems are present in two forms. One form is "cap stems" which are short and of small diameter and which originally served to attach individual grapes to their bunches. The other form is "vine stems". Vine stems are larger stems which, originally, united bunches of grapes and connected the bunches with their vines. After grapes are dried into raisins, stems of both forms have a substantially lower specific gravity than raisins. The sub-standard raisins are, frequently, of irregular shape and approach dried grape skins in consistency and are thus also of lower specific gravity than the desired raisins. Conventional shaking screens, which rely on gravity, as before stated, are not fully effective in removing cap stems and those sub-standard raisins which are smaller than desired raisins even though debris of these two types if of smaller dimensions than the desired raisins. Further, conventional shaking screens are ineffective for separating vine stems, which are often of a branching configuration and have at least one dimension which is at least equal to and, frequently greater than, any dimension of the desired raisins. It is also evident that conventional shaking screens cannot effectively separate those sub-standard raisins which, as is commonly the case, are of a flattened configuration and of greater dimensions than the desired raisins.
With raisins, the need for an improved cleaning system is greatest toward the end of the present mechanical cleaning process where the raisins are, in fact, marketable by present standards, but still contain a substantial amount of debris in the form of sub-standard raisins and vine stems. At present, manual picking of vine stems and the like is required to obtain raisins of the highest freedom from debris after mechanical cleaning is substantially complete. Such manual picking is not only relatively expensive but, at best, is relatively inefficient since it is easy to overlook the debris to be removed.